This invention relates to optical fiber lasers and amplifiers and more particularly to superfluorescent broadband fiber lasers.
Superfluorescent broadband doped fiber laser sources are described in U.S. Pat. Nos. : 4,637,025 to Snitzer, et, al.; 4,938,556 to Digonnet, et.al., which is a continuation in part of 4,859,016 to Shaw, et. al., which is a division of 4,723,824 which is a division of 4,674,830 (all for fiber optic amplifiers); and 5,136,600 to Fidric, et.al., which are incorporated herein by reference. The superfluorescent fiber lasers and amplifiers in the patents generally comprise an optical fiber having it's core or cladding or both doped with an active laser material and pumped at an appropriate frequency and sufficient intensity to produce amplified spontaneous emission (ASE). These patents disclose both single pass and double pass configurations. In a single pass configuration the output signal results from only one pass amplification of the spontaneous emission. In a double pass configuration a dichroic filter or a mirror and coupler are employed to reflect the backward spontaneous emission signal (not the pump signal) back through the doped fiber for second-pass amplification of the spontaneous emission. Unfortunately these superfluorescent sources are quite sensitive to optical feedback of the output signal in the doped fiber, which causes an onset of oscillating modes and a resultant narrowband emission. Such an oscillating condition prevents operating these devices as high power broadband light sources. An alternate approach is a resonant laser light source described in U.S. Pat. No. 4,964,131 to Liu, et. al. which is pumped at a wavelength selected to be outside the pump variable tuning range of the lasing material, e.g., Neodynium. This resonant laser has been found to be less efficient and considerably less stable wavelength wise in comparison to the referenced double pass fiber sources.
An object of this invention is the provision of an improved superfluorescent source. Another object is the provision of such a source that has an efficiency and a stability comparable to conventional double pass configurations and which has as high a resistance to optical feedback as the single pass approach. Another object is the provision of a source of polarization selective superfluorescent emission output light that can be operated on in a utilization device (e.g., interferometric sensors) and reapplied to the doped fiber for producing an amplified output signal having a prescribed polarization and that is the result of photons being amplified four times through the gain medium of the doped fiber.